Observations of accretion in black hole candidate x-RAY binaries and ultraluminous x-RAY sources have reached a point where they are capable of providing tight constraints on the properties of the…

This federal contract award, with an ID of 80NSSC18K1018, was granted by the National Aeronautics and Space Administration (NASA) to the Rector & Visitors of the University of Virginia (UVA), doing business as the University of Virginia. The $377,412.86 grant will fund research to improve radiation transfer models used to analyze observational data from X-ray satellites studying black hole accretion flows. The goal is to develop more accurate, frequency-dependent radiation transfer algorithms to better capture the physics of these systems and generate detailed spectral predictions that can be compared to observational data. This work is intended to support ongoing and future NASA missions studying black holes and high-energy astrophysical phenomena. The contract does not have a set-aside designation, indicating open competition. UVA has a history of federal research grants and contracts across various agencies, leveraging its expertise in fields like engineering, data science, and specialized scientific domains.

Observations of accretion in black hole candidate x-RAY binaries and ultraluminous x-RAY sources have reached a point where they are capable of providing tight constraints on the properties of the accretion flows and the black hole spacetimes in which they reside. At this point, a primary limitation is the quality of the models used to interpret the data. In order to provide better first-principles models, we propose to extend the capabilities of our radiation hydrodynamics code to include monte carlo based radiation transfer methods. This extension will allow US to better capture the frequency dependent aspects of the radiative transfer problems in our simulations and to provide detailed, simulation based predictions of the spectrum, variability, and polarization of black hole accretion flows. We have already developed state-of-the-art radiation transfer algorithms for solving the angle dependent transfer in the general relativistic magnetohydrodynamic code: athena++. These radiation transfer methods allow US to compute the radiation forces, heating, and cooling source terms in simulations of accretion flows. The methods have already been implemented in a special relativistic approach and we are in the process of extending the radiation transfer to fully general relativistic algorithms. The radiation transfer is solved in the simulations using a gray (frequency averaged) conservative finite difference scheme. This approach does not provide US with the frequency dependent information required to make detailed spectral predictions. Therefore, we proposed to develop a monte carlo based algorithm for solving the frequency dependent radiation transfer equation on hydrodynamics simulation grid. In the first stage of the project, we will integrate and the PI􀁉s existing monte carlo code into athena ++ and use IT to compute steady state spectra from snapshots of time dependent simulations. Next, we will extend the module to run in concert with the simulations to provide time-dependent spectral predictions. Finally, we will couple our monte carlo transfer scheme directly with the hydrodynamics. We will implement both a monte carlo only framework as well as a hybrid framework that couples the monte carlo solver to a conservative finite difference integrator of the relativistic stress-energy tensor in black hole spacetimes. The primary goals of this proposal are to improve the simulations by better modeling the frequency dependent properties of the radiation and to generate detailed spectral predictions. The first goal is particularly important for accurately modeling the heating and cooling effects due to compton scattering, which can depend sensitively on the local radiation spectrum. The second goal allows US to construct sophisticated synthetic spectra, which can be directly compared with data. We will create simulated observations using these outputs and analyze them using standard spectral analysis software and techniques (e.g. xspec). The results of this analysis will aid in interpreting the results from observations with current and former x-RAY satellites such as chandra, swift, nustar, XMM-newton, suzaku, as well as future NASA missions such as ixpe and xarm.